Non-Periodic Control Communications in Wireless and Other Process Control Systems

ABSTRACT

Disclosed is a controller having a processor and a control module adapted for periodic execution by the processor and configured to be responsive to a process variable to generate a control signal for a process. An iteration of the periodic execution of the control module involves implementation of a routine configured to generate a representation of a process response to the control signal. The routine is further configured to maintain the representation over multiple iterations of the periodic execution of the control module and until an update of the process variable is available. In some cases, the update of the process variable is made available via wireless transmission of the process signal. In those and other cases, the controller may be included within a process control system having a field device to transmit the process signal indicative of the process variable non-periodically based on whether the process variable has changed by more than a predetermined threshold. In some embodiments, the field device also transmits the process signal if a refresh time has been exceeded since a last transmission.

RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.11/258,676 filed Oct. 25, 2005 and entitled “Non-Periodic ControlCommunications in Wireless and Other Process Control Systems.”

TECHNICAL FIELD

The present invention relates generally to process control systems and,more particularly, to the transmission and processing of wireless and/ornon-periodic control communications in process control systems.

DESCRIPTION OF THE RELATED ART

Process control systems, such as distributed or scalable process controlsystems like those used in chemical, petroleum or other processes,typically include one or more process controllers communicativelycoupled to each other, to at least one host or operator workstation andto one or more field devices via analog, digital or combined analogdigital buses. The field devices, which may be, for example, valves,valve positioners, switches and transmitters (e.g., temperature,pressure and flow rate sensors), perform functions within the processsuch as opening or closing valves and measuring process parameters. Theprocess controller receives signals indicative of process measurementsmade by the field devices and/or other information pertaining to thefield devices, and uses this information to implement a control routineto generate control signals which are sent over the buses to the fielddevices to control the operation of the process. Information from thefield devices and the controller is typically made available to one ormore applications executed by the operator workstation to enable anoperator to perform any desired function with respect to the process,such as viewing the current state of the process, modifying theoperation of the process, etc.

Some process control systems, such as the DeltaV® system sold by FisherRosemount Systems, Inc., headquartered in Austin, Tex., use functionblocks or groups of function blocks referred to as modules located inthe controller or in different field devices to perform controloperations. In these cases, the controller or other device is capable ofincluding and executing one or more function blocks or modules, each ofwhich receives inputs from and/or provides outputs to other functionblocks (either within the same device or within different devices), andperforms some process operation, such as measuring or detecting aprocess parameter, controlling a device, or performing a controloperation, such as the implementation of aproportional-derivative-integral (PID) control routine. The differentfunction blocks and modules within a process control system aregenerally configured to communicate with each other (e.g., over a bus)to form one or more process control loops.

Process controllers are typically programmed to execute a differentalgorithm, sub-routine or control loop (which are all control routines)for each of a number of different loops defined for, or contained withina process, such as flow control loops, temperature control loops,pressure control loops, etc. Generally speaking, each such control loopincludes one or more input blocks, such as an analog input (AI) functionblock, a single-output control block, such as aproportional-integral-derivative (PID) or a fuzzy logic control functionblock, and an output block, such as an analog output (AO) functionblock. Control routines, and the function blocks that implement suchroutines, have been configured in accordance with a number of controltechniques, including PID control, fuzzy logic control, and model-basedtechniques such as a Smith Predictor or Model Predictive control (MPC).

To support the execution of the routines, a typical industrial orprocess plant has a centralized control room communicatively connectedwith one or more process controllers and process I/O subsystems, which,in turn, are connected to one or more field devices. Traditionally,analog field devices have been connected to the controller by two- orfour-wire current loops for both signal transmission and the supply ofpower. An analog field device that transmits a signal to the controlroom (e.g., a sensor or transmitter) modulates the current runningthrough the current loop, such that the current is proportional to thesensed process variable. On the other hand, analog field devices thatperform an action under control of the control room is controlled by themagnitude of the current through the loop.

More recently, field devices superimpose digital data on the currentloop used to transmit the analog signals. For example, the HighwayAddressable Remote Transducer (HART) protocol uses the loop currentmagnitude to send and receive analog signals, but also superimposes adigital carrier signal on the current loop signal to enable two-wayfield communication with smart field instruments. Another protocolgenerally referred to as Fieldbus defines two sub-protocols, onesupporting data transfers at a rate up to 31.25 kilobits per secondwhile powering field devices coupled to the network, and the othersupporting data transfers at a rate up to 2.5 megabits per secondwithout providing any power to field devices. With these types ofcommunication protocols, smart field devices, which are typically alldigital in nature, support a number of maintenance modes and enhancedfunctions not provided by older control systems.

With the increased amount of data transfer, one particularly importantaspect of process control system design involves the manner in whichfield devices are communicatively coupled to each other, to controllersand to other systems or devices within a process control system or aprocess plant. In general, the various communication channels, links andpaths that enable the field devices to function within the processcontrol system are commonly collectively referred to as an input/output(I/O) communication network.

The communication network topology and physical connections or pathsused to implement an I/O communication network can have a substantialimpact on the robustness or integrity of field device communications,particularly when the network is subjected to adverse environmentalfactors or harsh conditions. These factors and conditions can compromisethe integrity of communications between one or more field devices,controllers, etc. The communications between the controllers and thefield devices are especially sensitive to any such disruptions, inasmuchas the control routines typically require periodic updates of theprocess variables for each iteration of the routine. Compromised controlcommunications could therefore result in reduced process control systemefficiency and/or profitability, and excessive wear or damage toequipment, as well as any number of potentially harmful failures.

In the interest of assuring robust communications, I/O communicationnetworks used in process control systems have historically beenhardwired. But unfortunately, hardwired networks introduce a number ofcomplexities, challenges and limitations. For example, the quality ofhardwired networks may degrade over time. Moreover, hardwired I/Ocommunication networks are typically expensive to install, particularlyin cases where the I/O communication network is associated with a largeindustrial plant or facility distributed over a large area, for example,an oil refinery or chemical plant consuming several acres of land. Therequisite long wiring runs typically involve substantial amounts oflabor, material and expense, and may introduce signal degradationarising from wiring impedances and electromagnetic interference. Forthese and other reasons, hardwired I/O communication networks aregenerally difficult to reconfigure, modify or update.

It has been suggested to use wireless I/O communication networks toalleviate some of the difficulties associated with hardwired I/Onetworks. For example, U.S. Patent Publication No. 2003/0043052,entitled “Apparatus for Providing Redundant Wireless Access to FieldDevices in a Distributed Control System,” the entire disclosure of whichis hereby incorporated by reference, discloses a system utilizingwireless communications between controllers and field devices to augmentor supplement the use of hardwired communications.

Generally speaking, however, reliance on wireless communications forcontrol-related transmissions has been limited due to, among otherthings, reliability concerns. As described above, modern process controlrelies on reliable data communication between the controller and thefield devices to achieve optimum control levels. Moreover, typicalcontrollers execute control algorithms at fast rates to quickly correctunwanted deviations in the process. Unfortunately, environmental factorsor other conditions may create intermittent interferences that impede orprevent the fast communications necessary to support such execution ofcontrol algorithms.

Power consumption is another complicating factor for wirelesscommunications in process control. Disconnected from the I/O network,the field devices may need to provide their own power source.Accordingly, field devices may be battery powered, draw solar power, orpilfer ambient energy such as vibration, heat, pressure, etc. For thesedevices, energy consumed for data transmission may constitute asignificant portion of total energy consumption. In fact, more power maybe consumed during the effort to establish and maintain a wirelessconnection than during other important operations performed by the fielddevice, such as the steps taken to sense or detect the process variablebeing measured.

SUMMARY OF THE DISCLOSURE

In accordance with one aspect of the disclosure, a controller is usefulfor a process where a process signal is indicative of a process variableof the process. The controller includes a processor and a control moduleadapted for periodic execution by the processor and configured to beresponsive to the process variable to generate a control signal for theprocess. An iteration of the periodic execution of the control moduleinvolves and includes implementation of a routine configured to generatea representation of a process response to the control signal. Theroutine is further configured to maintain the representation overmultiple iterations of the periodic execution of the control module anduntil an update of the process variable is available.

In some cases, the update of the process variable is made available viawireless transmission of the process signal.

The control module may include a closed-loop control scheme thatutilizes the process response representation to determine the controlsignal. Alternatively or additionally, the routine includes a positivefeedback network to determine the process response representation basedon past values of the control signal. Alternatively or additionally, theroutine implements a filter algorithm to determine the process responserepresentation.

In some cases, the routine is further configured for implementationbased on non-periodic updates of the process variable. Alternatively oradditionally, the process response representation may include a processvariable response representation, and the routine may be furtherconfigured to update the process variable response representation oncethe update of the process variable is available. The routine may thendetermine the expected response of the process based on a last update ofthe process variable, the control signal, and an elapsed time since thelast update of the process variable. The routine may also determine anupdated reset contribution based on the expected response of the processto the last update and the elapsed time since the last update. Theexpected process response may include a model that includes process ormeasurement delay.

In accordance with another aspect of the disclosure, a process controlsystem includes a field device to transmit a process signal indicativeof a process variable of a process, and a controller in communicationwith the field device to receive an update of the process variable viathe process signal and generate a control signal for the process. Thecontroller has a processor and a control module adapted for periodicexecution by the processor. The field device wirelessly transmits theprocess signal non-periodically based on whether the process variablehas changed by more than a predetermined threshold.

In some embodiments, the field device transmits the process signal if arefresh time has been exceeded since a last transmission.

The routine may be further configured to maintain a process responserepresentation, which may be generated by a routine implemented via theperiodic execution of the control module, over multiple iterations ofthe periodic execution of the control module and until the field devicetransmits the process signal. The control module may include aclosed-loop control scheme that utilizes the process responserepresentation to determine the control signal. Alternatively oradditionally, the routine may include a positive feedback network todetermine the process response representation based on past values ofthe control signal. Alternatively or additionally, the routine mayimplement a filter algorithm to determine the process responserepresentation. Alternatively or additionally, the routine may befurther configured for implementation based on non-periodic updates ofthe process variable. The process response representation may include aprocess variable response representation, where the routine is furtherconfigured to update the process variable response representation oncethe update of the process variable is available. The routine may thendetermine the expected response of the process based on a last update ofthe process variable, the control signal, and an elapsed time since thelast update of the process variable.

In accordance with yet another aspect of the disclosure, a method ofcontrolling a process includes implementing a process control routine togenerate a control signal for the process based on a process variable,and detecting whether an update of the process variable is available.The implementation of the process control routine includes or involvesgenerating a representation of a process response to the control signal,and maintaining the process response representation over multipleiterations of the implementing step until the update of the processvariable is detected.

In some cases, the method further includes or involves receiving awireless transmission of a process signal indicative of the update ofthe process variable.

The implementation of the process control routine further includes orinvolves executing a closed-loop control scheme that utilizes theprocess response representation to determine the control signal.

In some cases, the process response representation includes a processvariable response representation, such that the implementation of theprocess control routine further includes or involves updating theprocess variable response representation once the update of the processvariable is available. The control routine implementation may theninclude or involve determining the expected response of the processbased on a last update of the process variable, the control signal, andan elapsed time since the last update of the process variable. Thecontrol routine implementation may further include or involvedetermining an updated reset contribution based on the expected responseof the process to the last update and the elapsed time since the lastupdate.

In accordance with another aspect of the disclosure, a method is usefulfor controlling a process having a process variable. The method includesor involves receiving wirelessly a process signal to obtain an update ofthe process variable, and implementing periodically a process controlroutine to generate a control signal for the process based on theprocess signal. The receiving step occurs non-periodically such that theprocess control routine is configured to utilize non-periodic updates ofthe process variable received due to the process variable changing bymore than a predetermined threshold or due to a time elapsed since aprior update of the process variable.

In some cases, the implementing step includes or involves executing aroutine configured to generate a representation of a process response tothe control signal, and maintaining the process response representationover multiple iterations of the implementing step and until the updateof the process variable is available. The process control routine mayinclude a closed-loop control scheme that utilizes the process responserepresentation to determine the control signal. The process responserepresentation may include a process variable response representation,such that the implementing step further includes or involves the step ofupdating the process variable response representation once the update ofthe process variable is available. The implementing step may furtherinclude or involve determining the expected response of the processbased on a last update of the process variable, the control signal, andan elapsed time since the last update of the process variable. Theimplementing step may still further include or involve determining anupdated reset contribution based on the expected response of the processto the last update and the elapsed time since the last update.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the disclosure, reference should bemade to the following detailed description and accompanying drawingfigures, in which like reference numerals identify like elements in thefigures, and in which:

FIG. 1 is a schematic representation of a process control system havinga controller configured to implement one or more control routinesutilizing non-periodic or less frequent control communicationstransmitted via hardwired connections between the controller and anumber of field devices in accordance with one aspect of the disclosure;

FIG. 2 is a graphical representation of the implementation of a controlroutine by the controller of FIG. 1 via a plot depicting a processresponse to a process input and exemplary instances of measurementtransmissions and control execution iterations;

FIG. 3 is a schematic representation of a process control system havinga controller configured to implement one or more control routinesutilizing non-periodic or less frequent control communicationstransmitted via wireless connections between the controller and a numberof field devices in accordance with one aspect of the disclosure;

FIG. 4 is a schematic representation of the controller of either FIG. 1or FIG. 3 in accordance with one embodiment in which the controllergenerates a process input signal to control a process despite wireless,non-periodic or other transmission of process measurements lessfrequently than the control execution rate; and,

FIG. 5 is a schematic representation of a controller in accordance withan alternative embodiment configured to control a process having processand/or measurement delay.

While the disclosed system and method are susceptible of embodiments invarious forms, there are illustrated in the drawing (and will hereafterbe described) specific embodiments of the invention, with theunderstanding that the disclosure is intended to be illustrative, and isnot intended to limit the invention to the specific embodimentsdescribed and illustrated herein.

DESCRIPTION OF PREFERRED EMBODIMENTS

Disclosed herein are a process control system and method that implementcommunication and control techniques to support the transfer of processcontrol data between a controller and field devices, such astransmitters and other instrumentation. In another aspect of thedisclosure, the disclosed techniques enable the process measurements andother information gathered by the field devices to be used by thecontroller in the implementation of one or more process controlroutines.

In the past, such measurements were obtained and transmitted to thecontroller on a regularly timed, or periodic, basis to ensure thatupdated data was available for each iteration of the periodic executionof the process control routines. In contrast, the disclosed techniquesallow the transmission of such data to be non-periodic and/or atintervals longer than the control execution period. As a result, thedisclosed techniques may be well-suited for supporting process controlmeasurements that may be provided less frequently or non-regularly.Non-regular or less frequent transmissions may be advantageous for anumber of reasons, and may result from any number of factors, conditionsor aspects of the process control system or its environment.

In accordance with some embodiments, the disclosed techniques areutilized in connection with communication schemes, such as wirelesscommunications, involving process control data transmissions made on areport-by-exception basis. Exception reporting of the process controldata in a wireless communication context may present a number ofadvantages. For example, the rate at which power is consumed in thefield by the transmitters or other field devices may be lowered, therebyconserving battery power or other limited power supplies.

Unlike past exception reporting, however, the disclosed techniquessupport the transmission of data utilized in a process control routineexecuted on a periodic basis. And despite the admonitions of the pastdiscouraging the execution of process control routines utilizing dataprovided on an event-triggered basis, practice of the disclosedtechniques accommodates the periodic execution of process controlroutines without detrimental sacrifices in performance.

Although well suited for, and described at times herein in connectionwith, wireless communication schemes, practice of the disclosedtechniques is not limited to any particular communication scheme,context, or protocol, or any process control network, architecture,controller or system. Instead, the disclosed techniques may be appliedin any number of contexts in which process control data is transmittedless frequently than the control routine execution period, and for anydesired reason. Accordingly, the following description is set forth withthe understanding that practice of the disclosed techniques is notlimited to the low-power wireless communication context described below.

Referring now to FIG. 1, a process control system 10 includes a processcontroller 11 connected to a data historian 12 and to one or more hostworkstations or computers 13 (which may be any type of personalcomputers, workstations, etc.), each having a display screen 14. Thecontroller 11 is also connected to field devices 15-22 via input/output(I/O) cards 26 and 28. The data historian 12 may be any desired type ofdata collection unit having any desired type of memory and any desiredor known software, hardware or firmware for storing data. The datahistorian 12 may be separate from (as illustrated in FIG. 1) or a partof one of the workstations 13. The controller 11, which may be, by wayof example, the DeltaV controller sold by Fisher-Rosemount Systems,Inc., is communicatively connected to the host computers 13 and to thedata historian 12 via, for example, an ethernet connection or any otherdesired communication network. The controller 11 is also communicativelyconnected to the field devices 15-22 using either a hardwired orwireless communication scheme, as described further herein. In eithercase, any desired hardware, software and firmware may be utilized toimplement the schemes, which may associated with, for example, standard4-20 ma devices (when hardwired) and/or any smart communication protocolsuch as the FOUNDATION Fieldbus protocol, the HART protocol, etc. In theexemplary embodiment shown in FIG. 1, however, communications betweenthe controller 11 and the field devices 15-22 involve hardwiredconnections.

More generally, the field devices 15-22 may be any types of devices,such as sensors, valves, transmitters, positioners, etc., while the I/Ocards 26 and 28 may be any types of I/O devices conforming to anydesired communication or controller protocol. In the embodimentillustrated in FIG. 1, the field devices 15-18 are standard 4-20 madevices that communicate over analog lines to the I/O card 26, while thefield devices 19-22 are smart devices, such as Fieldbus field devices,that communicate over a digital bus to the I/O card 28 using Fieldbusprotocol communications. Of course, the field devices 15-22 may conformto any other desired standard(s) or protocols, including any standardsor protocols developed in the future.

The controller 11 includes a processor 23 that implements or overseesone or more process control routines (or any module, block, orsub-routine thereof) stored in a memory 24. The process control routinesstored in the memory 24 may include or be associated with control loopsstored therein. Generally speaking, the controller 11 communicates withthe devices 15-22, the host computers 13 and the data historian 12 tocontrol a process in any desired manner. It should be noted that anycontrol routines or modules described herein may have parts thereofimplemented or executed by different controllers or other devices if sodesired. Likewise, the control routines or modules described herein tobe implemented within the process control system 10 may take any form,including software, firmware, hardware, etc. For the purpose of thisdisclosure, a control module may be any part or portion of a processcontrol system including, for example, a routine, a block or any elementthereof, stored on any computer readable medium. Control routines, whichmay be modules or any part of a control procedure such as a subroutine,parts of a subroutine (such as lines of code), etc. may be implementedin any desired software format, such as using object orientedprogramming, using ladder logic, sequential function charts, functionblock diagrams, or using any other software programming language ordesign paradigm. Likewise, the control routines may be hard-coded into,for example, one or more EPROMs, EEPROMs, application specificintegrated circuits (ASICs), or any other hardware or firmware elements.Still further, the control routines may be designed using any designtools, including graphical design tools or any other type ofsoftware/hardware/firmware programming or design tools. Thus, thecontroller 11 may be configured to implement a control strategy orcontrol routine in any desired manner.

In some embodiments, the controller 11 implements a control strategy orscheme using what are commonly referred to as function blocks, whereeach function block is an object or other part (e.g., a subroutine) ofan overall control routine that operates in conjunction with otherfunction blocks (via communications called links) to implement processcontrol loops within the process control system 10. Function blockstypically perform one of an input function, such as that associated witha transmitter, a sensor or other process parameter measurement device, acontrol function, such as that associated with a control routine thatperforms PID, fuzzy logic, etc. control, or an output function whichcontrols the operation of some device, such as a valve, to perform somephysical function within the process control system 10. Of course,hybrid and other types of function blocks exist and may be utilizedherein. The function blocks may be stored in and executed by thecontroller 11, which is typically the case when the function blocks areused for, or are associated with standard 4-20 ma devices and some typesof smart field devices such as HART devices. Alternatively oradditionally, the function blocks may be stored in and implemented bythe field devices themselves, which may be the case with Fieldbusdevices. While the description of the control system 10 is providedherein using a function block control strategy, the disclosed techniquesand system may also be implemented or designed using other conventions,such as ladder logic, sequential function charts, etc. or using anyother desired programming language or paradigm.

As illustrated by the exploded block 30 of FIG. 1, the controller 11 mayinclude a number of single-loop control routines, illustrated asroutines 32 and 34, and, if desired, may implement one or more advancedcontrol loops, illustrated as control loop 36. Each such loop istypically referred to as a control module. The single-loop controlroutines 32 and 34 are illustrated as performing single loop controlusing a single-input/single-output fuzzy logic control block and asingle-input/single-output PID control block, respectively, connected toappropriate analog input (AI) and analog output (AO) function blocks,which may be associated with process control devices such as valves,with measurement devices such as temperature and pressure transmitters,or with any other device within the process control system 10. Theadvanced control loop 36 is illustrated as including an advanced controlblock 38 having inputs communicatively connected to one or more AIfunction blocks and outputs communicatively connected to one or more AOfunction blocks, although the inputs and outputs of the advanced controlblock 38 may be connected to any other desired function blocks orcontrol elements to receive other types of inputs and to provide othertypes of control outputs. The advanced control block 38 may implementany type of multiple-input, multiple-output control scheme, and mayconstitute or include a model predictive control (MPC) block, a neuralnetwork modeling or control block, a multi-variable fuzzy logic controlblock, a real-time-optimizer block, etc. It will be understood that thefunction blocks illustrated in FIG. 1, including the advanced controlblock 38, can be executed by the controller 11 or, alternatively, can belocated in and executed by any other processing device, such as one ofthe workstations 13 or one of the field devices 19-22.

With reference now to FIG. 2, the implementation of each of the controlloops 32, 34 and 36 is generally adapted for periodic execution viamultiple iterations 40 of the control routine. In a conventional case,each iteration 40 is supported by an updated process measurement 42provided by, for instance, a transmitter or other field device. To avoidthe restrictions of synchronizing the measurement value with thecontrol, many past controllers (or control loops) were designed toover-sample the measurement by a factor of 2-10 times. Suchover-sampling helped ensure that the process measurement was current foruse in the control scheme. Also, to minimize control variation,conventional designs specified that feedback control should be executed4-10 times faster than the process response time, which is depicted inFIG. 2 as a process time constant plus a process delay after a stepchange 44 in the process input. More generally, the process response isindicated by a change in a process output or variable 46 over time.Thus, to satisfy these conventional design requirements, the measurementvalue has often been sampled much faster that the process responds, asillustrated in FIG. 2.

Generally speaking, the disclosed techniques address the challenge oftransmitting the measurement values at such high rates. For example, andas described above, the sensing functionality associated with themeasurement may not consume much of the power supply for the sensor ortransmitter, but the transmission of the measurement value via awireless communication link may, over time, constitute a significantpower supply drain. Even if measurement and control execution aresynchronized, as in Foundation Fieldbus control schemes, theconventional approach to scheduling control 4-10 times faster than theprocess response may still result in too much power consumption duringdata transmission. Thus, to reduce transmitter power consumption thedisclosed techniques generally support minimizing how often ameasurement value is communicated.

To that end, and in accordance with one aspect of the disclosure, thedisclosed techniques generally configure the process control system 10,and the controller 11 and transmitting and other field devices thereof,to transmit a new measurement value on a non-periodic basis when certainconditions are satisfied. In one embodiment, a new measurement value istransmitted based on whether the process variable has changed by morethan a predetermined threshold (e.g., an amount determined to besignificant). More specifically, if the magnitude of the differencebetween the new measurement value and the last communicated measurementvalue is greater that a specified resolution, then a trigger may begenerated such that the measurement will be updated.

In other cases, a new measurement value is transmitted when thedifference exceeds the specified resolution (as in the prior case), aswell as when the time since the last communication exceeds apredetermined refresh time. In other words, either a change in theprocess variable (e.g., the process response between control executioniterations 48 and 50), or the passing of a default time (e.g., the timeelapsed between iterations 52 and 54), may result in a measurementtransmission. The refresh, or default, time for measurement transmissionmay vary between control loops, inasmuch more or less frequent updatesmay be suitable depending on whether the process is slow moving or rapidin response (as indicated, for instance, by the process time constant).In some cases, a determination may be made during the tuning of thecontrol loop based on the time constant, and adjusted thereafter asdesired. In any case, the default or refresh time acts as an integritycheck, or override, after periods of time without a measurement update.Such checks may be useful to, for instance, facilitate the final driveof the process variable to target.

In the meantime, the transmitter, sensor or other field deviceresponsible for obtaining the measurement values may still beperiodically sampling the measurement at any desired rate, such as theconventional 4-10 times the process response time. The disclosedtechniques then determine whether the sampled values are transmitted tothe controller 11.

FIG. 3 depicts an exemplary case in which the disclosed techniques maybe applied to reduce power consumption during wireless communication ofprocess control data to support the operation of the controller 11 and,more generally, the process control system 10 of FIG. 1. At the outset,however, it should be noted that the hardwired connections shown inFIGS. 1 and 3 may also utilize and benefit from application of thedisclosed techniques. For example, one or more of the hardwired devicesmay also rely on a limited power supply or otherwise benefit fromreduced data transmission. In one exemplary case, the system 10 mayinclude a sampled analyzer or other sampling system designed to providemeasurement data at rates slower than the control execution rate.

It should further be noted that, for ease in illustration, a number ofwireless field devices have been added to the process control system 10,with the field devices 15-22 remaining hardwired to the controller 11via the I/O devices 26 and 28. In alternative embodiments, one or moreof the field devices 15-22 may also or alternatively communicate withthe controller 11 wirelessly in accordance with the disclosedtechniques.

In the exemplary case shown in FIG. 3 however, the disclosed techniquesgenerally involve the wireless transmission of data measured or sensedby transmitters 60-64. The wireless communications may be establishedusing any desired equipment, including hardware, software, firmware, orcombination thereof now known or later developed. The exemplaryequipment of this embodiment is represented by an antenna 65 coupled anddedicated to the transmitter 60 and a wireless router or other module 66having an antenna 67 to collectively handle communications for thetransmitters 61-64. In some cases, the transmitters 60-64 may constitutethe sole link between the process sensors and the control room and, assuch, be relied upon to send accurate signals to the control network toensure that product quality and flow are not compromised. Thus, thetransmitters 60-64, often referred to as process variable transmitters(PVTs), may play a significant role in the process control system 10.

On the receiving end of the wireless communication links, the controller11 may have one or more I/O devices 68 and 70 with respective antennas72 and 74. More generally, practice of the disclosed techniques is notlimited to any configuration of transmitters or wireless equipment.

Each of the transmitters 60-64 or other field devices transmits aprocess signal indicative of a respective process variable (e.g., aflow, pressure, temperature or level) to the controller 11 for use inone or more control loops or routines. Generally speaking, thecontroller 11 may include a number of elements directed to supportingthe wireless communication and, specifically, reception, of the processsignals. The elements may include or constitute, for example, softwareroutines stored in the memory 24 or hardware or firmware residentelsewhere in the controller 11. In any case, the manner in which thewireless communications are received (e.g. demodulated, decoded, etc.)may take any desired form, and will only be generally addressed herein.In one example, the controller 11 may include a communications stack 80to process the incoming signals, and a module or routine 82 to detectwhen an incoming signal has provided a measurement update. The detectionroutine 82 may then generate a flag or other signal to denote that databeing provided via the communications stack 80 includes a newmeasurement value or measurement update. The new data and the updateflag may then be provided to one or more control modules 84 to beimplemented as discussed above in connection with the routines generallyshown in FIG. 1 and described in further detail below.

In some cases, the communications stack 80 and the update detectionmodule 82 are implemented by one or more of the I/O devices 26, 28, 68and 70 (FIGS. 1 and 3). Furthermore, the manner in which the updatedetection module 82 makes its determination may involve hardware,software, firmware or any combination thereof, and may involve anysuitable routine for comparing values of the process variable.

The communication techniques described above for the wireless (or other)transmitters generally result in non-periodic, irregular or otherwiseless frequent data transmissions. However, the communication ofmeasurement values from the field to the controller 11 has traditionallybeen structured to report in a periodic manner to, in turn, support theperiodic execution of the control routine(s). In other words, thecontrol routines are generally designed for, and rely on, periodicupdates of the measurement values.

To accommodate the non-periodic measurement updates, another aspect ofthe disclosure is generally directed to modifying or re-structuring thecontrol routine(s). In this manner, the process control system 10 mayrely on non-periodic or other updates that occur less frequently thanthe control execution period. And as a result, the disclosed techniquesgenerally support a form of exception reporting for the process variablemeasurements despite the periodic execution of the process controlroutines.

In fact, the underlying assumption in the control design (e.g., using ztransform, difference equations) and digital implementation of thecontrol routines, such as proportional-integral-derivative (PID)control, is that the algorithm is executed on a periodic basis. If themeasurement is not updated, then steps such as the integral (or reset)portion or contribution of the routine may not be appropriate. Forexample, if the control algorithm continues to execute using the last,outdated measurement value, then the output will continue to move basedon the reset tuning and error between the last measured value and thesetpoint. On the other hand, if the control routine is only executedwhen a new measurement is communicated, then the control response tosetpoint changes and feedforward action on measured disturbances couldbe delayed. Control routines may also include calculations based on thetime elapsed since the last iteration. But with non-periodic and/or lessfrequent measurement transmissions, calculating the reset contributionbased on the control execution period (i.e., the time since the lastiteration) may result in increased process variability.

In view of the foregoing challenges, and to provide accurate andresponsive control when measurement values are not updated on a periodicbasis, disclosed herein are control techniques that generally modify theprocess control routine based on whether an update of the processvariable is available. In some cases, the control routine may berestructured in accordance with the disclosed techniques based on theexpected process response since the last measurement update.

An exemplary embodiment of a control scheme configured in accordancewith one aspect of the disclosed techniques is shown in FIG. 4, wherethe process is generally and schematically indicated at 100. Theexemplary control scheme may correspond with a component 102 (or set ofcomponents, as desired) of the controller 11 configured to provide thefunctionality of the communications stack 80, the update detectionmodule 82 and the control module 84 also shown and described inconnection with FIG. 3. Generally speaking, the controller 11 receives asetpoint from, for example, one of the workstations 13 (FIG. 1) or fromany other source within or in communication with the process controlsystem 10 to generate one or more process input or other control signalsto control the process 100, which may be subjected to measured orunmeasured disturbances schematically shown at 104. As described above,the process input signal(s) may control an actuator associated with avalve or any other field device to effect a response in the operation ofthe process. The process response to changes in the process input signalare measured or sensed by a transmitter, sensor or other field device106, which may, for example, correspond, for example, with any one ofthe transmitters 60-64 shown in FIG. 3. As a result, the communicationlink (depicted via dashed lines) between the transmitter 106 and thecontroller 11 may include a wireless connection. Alternatively, or inaddition, the communications may include a hardwired connection, asdesired, which may benefit from the disclosed techniques because, forexample, it is intermittently available or operational.

In this exemplary case, the controller 11 implements a singleclosed-loop control routine, such as a PI control routine. Accordingly,the control loop includes several standard PI control scheme elements,including a summing point 108 for comparing the setpoint with theprocess variable data, a proportional gain element 110, another summingpoint 112 for combining, for instance, the proportional and integralcontributions, and a high-low limiter 114. In addition to the standardelements of the control scheme, this embodiment of the disclosed controltechnique utilizes a modified filter 116 to provide an indication of theexpected process response to the control signal. In this exemplary case,the expected process response is approximated as first order and isrealized by the modified filter included in the positive feedback loopthat determines the integral contribution of the PI control scheme. Moregenerally, the expected process response utilized in the controlimplementation may be provided by any model of the process, and is notlimited to incorporation in a positive feedback loop, a filter or anintegral or reset contribution. For example, the control utilizing amodel to provide the expected process response may incorporate aderivative contribution such that the control routine implements a PIDcontrol scheme.

The modified filter 116 differs from a traditional reset or integralcontribution in a number of ways. By way of background, a traditional PIcontroller may be implemented using a positive feedback network todetermine the reset contribution. Mathematically, it can be shown thatthe transfer function for the traditional implementation is equivalentto the standard formulation for unconstrained control i.e. output notlimited.

$\frac{O(s)}{E(s)} = {K_{P}\left( {1 + \frac{1}{{sT}_{Reset}}} \right)}$

-   -   where        -   K_(p)=Proportional Gain        -   T_(Reset)=Reset, seconds            One advantage of the positive feedback network is that the            reset contribution is automatically prevented from winding            up when the controller output is high or low limited, i.e.,            by the limiter 114.

In accordance with one aspect of the disclosure, the control techniqueimplemented by the disclosed system and method involves using anon-periodic measurement update of the process variable. The positivefeedback network of reset contribution (or other filter or routine) ismodified to accommodate such updates. Specifically, the filter 116 (orother routine) is configured such that the last calculated filter outputis maintained until a new measurement is communication (e.g., received).When a new measurement is received, the filter 116 calculates the newfilter output based on the last controller output (i.e., the controlsignal) and the elapsed time since a new measurement value wascommunicated. An exemplary case of this control technique is set forthbelow:

$F_{N} = {F_{N - 1} + {\left( {O_{{N - 1}\;} - F_{N - 1}} \right)*\left( {1 - ^{\frac{{- \Delta}\; T}{T_{Reset}}}} \right)}}$

where

F_(N)=New filter output

F_(N-1)=Filter output last execution=filter output after last newmeasurement

O_(N-1)=Controller output last execution

Δ=Elapsed time since a new value was communicated

In this way, the control routine accounts for the expected processresponse to the last measurement transmission when calculating thecontrol input based on the new measurement. And as a result, thetransmitter may implement any communication techniques in which anupdate is not provided for every iteration of the control execution,such as the techniques described above. For those communicationtechniques involving wireless transmissions, this allows wirelesstransmitters and other devices to minimize the amount of power consumedas a result of data transfer for process control.

It should be noted that the reset contribution of a closed-loop controlroutine such as that described above may provide an accuraterepresentation of the process response in a number of ways, such as ifthe process exhibits steady-state behavior. Other processes, such asdeadtime dominant processes, may involve the incorporation of additionalcomponents in the routine modeling the expected process response, asdiscussed below. But with regard to processes well represented by afirst-order model, the process time constant may be used to determinethe reset time for the PI (or PID) controller. More specifically, if onesets the reset time equal to the process time constant, the resetcontribution generally cancels out the proportional contribution suchthat, over time, the routine reflects the expected process response.This approach is reflected in the exemplary embodiment of FIG. 4 inwhich the reset contribution is effected by a positive feedback networkhaving a filter with the same time constant as the process timeconstant. While other models may be utilized, the positive feedbacknetwork, filter, or model provides a convenient mechanism fordetermining the expected response of a process having a known orapproximated process time constant.

As an example, the number of communications during the duration of atest involving the disclosed techniques was reduced by over 96% when therules for wireless communication were followed. The impact ofnon-periodic measurement updates on control performance was alsominimized through the use of the above-described, modified PI algorithm.Specifically, the difference in control performance is shown below inTable 1 in a comparison of the Integral Absolute Error (IAE) forperiodic measurement update vs. non-periodic updates.

TABLE 1 CONTROL PERFORMANCE DIFFERENCE Number of Communications/ControlCommunications IAE Periodic/standard PI controller 692 123 DisclosedTechniques 25 159 (Non-periodic communication with modified PI control)

For those processes that require PID control, the rate contribution tothe PID output may also be recomputed and updated only when a newmeasurement is received. In those cases, the derivative calculation maysimilarly use the elapsed time since the last new measurement.

As shown in FIG. 4, the communications stack 80 and, in someembodiments, the update detection module 82 (FIG. 3), process theincoming data from the transmitter 106 to generate a new value flag forthe modified filter 116. The new value flag is provided to the modifiedfilter 116 to determine when the new filter output should be calculated.

Referring now to FIG. 5, an alternative controller 120 configured inaccordance with the disclosed control techniques is similar in manyrespects to the controller 11 shown in FIG. 4. As a result, elementscommon to both controllers are identified with like reference numerals.The controller 120, however, incorporates an additional element into theroutine that determines the expected process response betweenmeasurement transmissions. In this case, the process may becharacterized as having a considerable amount of deadtime and, as aresult, a unit or block 122 is included in the model for deadtimecompensation. The incorporation of the deadtime unit 122 generally helpsto arrive at a more accurate representation of the process response.More specifically, the deadtime unit 122 may be implemented in anydesired fashion and may include or utilize methods common to Smithpredictors or other known control routines.

As shown by the above-described embodiments, the feedback, filter orother routine responsible for determining the expected process responseto the control signal may involve any type of model, network or otherarrangement of process control elements that help remove any offset orother error from the remainder of the process control routine. In thisway, the disclosed techniques are well suited for a variety of differentprocesses, and are not limited to those that exhibit first-orderbehavior. Quite to the contrary, the disclosed techniques are applicablein contexts in which different models, filters or blocks are involved indetermining the expected process response, and need not be limited touse in situations where the process model is highly accurate.

As described above, the disclosed techniques support a process controlconfiguration that avoids the need for oversampling process variables,thereby facilitating the use of wireless communications and othertransmitter scenarios where measurement values may not be availableregularly or as often as the control execution period. In short, thedisclosed techniques avoid having to constantly transmit measurementdata for process control routine execution. As a result of the disclosedchanges in the transmitter (or other field device) design and controlmodifications, measurement values are generally transmitted to onlycommunicate significant changes (from the last communicated value) orafter a refresh time. As a result, both the frequency of the transmittercommunications and the amount of power used for data transmission dropsignificantly.

Practice of the disclosed methods, system and techniques is not limitedto any one particular wireless architecture or communication protocol.Suitable exemplary architectures and communication support schemes aredescribed U.S. patent application Ser. No. 11/156,215 entitled “WirelessArchitecture and Support for Process Control Systems,” which was filedon Jun. 17, 2005, the entire disclosure of which is hereby incorporatedby reference. In fact, the disclosed modifications to the controlroutines are well-suited for any context in which the control routine isimplemented in a periodic manner, but without process variablemeasurement updates for each control iteration. Other exemplary contextsinclude where a sampled value is provided irregularly or more seldom by,for instance, an analyzer or via lab samples.

Practice of the disclosed technique is not limited to use withsingle-input, single-output PI or PID control routines, but rather maybe applied in a number of different multiple-input and/ormultiple-output control schemes and cascaded control schemes. Moregenerally, the disclosed technique may also be applied in the context ofany closed-loop model-based control routine involving one or moreprocess variables, one or process inputs or other control signals, suchas model predictive control (MPC).

The term “field device” is used herein in a broad sense to include anumber of devices or combinations of devices (i.e., devices providingmultiple functions, such as a transmitter/actuator hybrid), as well asany other device(s) that perform(s) a function in a control system. Inany event, field devices may include, for example, input devices (e.g.,devices such as sensors and instruments that provide status, measurementor other signals that are indicative of process control parameters suchas, for example, temperature, pressure, flow rate, etc.), as well ascontrol operators or actuators that perform actions in response tocommands received from controllers and/or other field devices.

When implemented, any of the software described herein may be stored inany computer readable memory such as on a magnetic disk, a laser disk,or other storage medium, in a RAM or ROM of a computer or processor,etc. Likewise, this software may be delivered to a user, a process plantor an operator workstation using any known or desired delivery methodincluding, for example, on a computer readable disk or othertransportable computer storage mechanism or over a communication channelsuch as a telephone line, the Internet, the World Wide Web, any otherlocal area network or wide area network, etc. (which delivery is viewedas being the same as or interchangeable with providing such software viaa transportable storage medium). Furthermore, this software may beprovided directly without modulation or encryption or may be modulatedand/or encrypted using any suitable modulation carrier wave and/orencryption technique before being transmitted over a communicationchannel.

While the present invention has been described with reference tospecific examples, which are intended to be illustrative only and not tobe limiting of the invention, it may be apparent to those of ordinaryskill in the art that changes, additions or deletions may be made to thedisclosed embodiments without departing from the spirit and scope of theinvention.

1. A process control system for a process having a process variable,comprising: a field device to transmit a process signal indicative ofthe process variable of the process; and, a controller in communicationwith the field device to receive an update of the process variable viathe process signal and generate a control signal for the process, thecontroller having a processor and a control module adapted for periodicexecution by the processor; wherein the field device wirelesslytransmits the process signal non-periodically based on whether theprocess variable has changed by more than a predetermined threshold. 2.The process control system of claim 1, wherein the field devicetransmits the process signal if a refresh time has been exceeded since alast transmission.
 3. The process control system of claim 1, wherein theperiodic execution of the control module implements a routine configuredto generate a representation of a process response to the controlsignal.
 4. The process control system of claim 3, wherein the routine isfurther configured to maintain the process response representation overmultiple iterations of the periodic execution of the control module anduntil the field device transmits the process signal.
 5. The processcontrol system of claim 4, wherein the control module comprises aclosed-loop control scheme that utilizes the process responserepresentation to determine the control signal.
 6. The process controlsystem of claim 4 wherein the routine comprises a positive feedbacknetwork to determine the process response representation based on pastvalues of the control signal.
 7. The process control system of claim 4,wherein the routine implements a filter algorithm to determine theprocess response representation.
 8. The process control system of claim4, wherein the routine is further configured for implementation based onnon-periodic updates of the process variable.
 9. The process controlsystem of claim 4, wherein the process response representation comprisesa process variable response representation, and wherein the routine isfurther configured to update the process variable responserepresentation once the update of the process variable is available. 10.The process control system of claim 9, wherein the routine determinesthe expected response of the process based on a last update of theprocess variable, the control signal, and an elapsed time since the lastupdate of the process variable.
 11. A method of controlling a processhaving a process variable, the method comprising the steps of: receivingwirelessly a process signal to obtain an update of the process variable;and, implementing periodically a process control routine to generate acontrol signal for the process based on the process signal; wherein thereceiving step occurs non-periodically such that the process controlroutine is configured to utilize non-periodic updates of the processvariable received due to the process variable changing by more than apredetermined threshold or due to a time elapsed since a prior update ofthe process variable.
 12. The method of claim 11, wherein theimplementing step comprises the steps of executing a routine configuredto generate a representation of a process response to the controlsignal, and maintaining the process response representation overmultiple iterations of the implementing step and until the update of theprocess variable is available.
 13. The method of claim 12, wherein theprocess control routine comprises a closed-loop control scheme thatutilizes the process response representation to determine the controlsignal.
 14. The method of claim 13, wherein the process responserepresentation comprises a process variable response representation, andwherein the implementing step further comprises the step of updating theprocess variable response representation once the update of the processvariable is available.
 15. The method of claim 14, wherein theimplementing step further comprises the step of determining the expectedresponse of the process based on a last update of the process variable,the control signal, and an elapsed time since the last update of theprocess variable.
 16. The method of claim 15, wherein the implementingstep further comprises the step of determining an updated resetcontribution based on the expected response of the process to the lastupdate and the elapsed time since the last update.